Digital to analog conversion circuitry is widely used, for example to implement one or more digital to analog converters (DACs) for Wi-Fi, OFDM and similar transmitters, which may be used in mobile telecommunication applications. For example, in an I/Q transmitter, one DAC (I-DAC) generates the analog in-phase (I) component of the signal and one DAC (Q-DAC) generates the analog quadrature (Q) component of the signal. The trajectory of the signal to be transmitted (e.g. of a Wi-Fi signal) can be visualized in an I/Q diagram showing a continuous line representing the evolution of the transmitted signal in time which is generated using the two DACs. The signal trajectory of a complex signal crosses one of the axes repeatedly, each time causing one of the DACs to provide an analog signal with alternating sign. In particular, every time the trajectory crosses the x-axis (I) or the y-axis (Q), a zero crossing or sign change occurs in the I-DAC or in the Q-DAC, respectively. Consequently, a lot of sign changing events take place within the DACs during the transmission of a Wi-Fi, OFDM or similar signal.
Some DACs used to generate the I- and Q-components of the signal (RF DACs) comprise an array of cells, which can be connected either to a local oscillator signal (LO) or to ground. The energy of the signal generated by an individual DAC depends on the number of cells which are simultaneously providing the LO signal. In a synchronous implementation of the RF DAC, data (i.e. the signal selecting a cell to provide the LO signal) changes only when the cell is not transparent for the LO signal (depending on the particular implementation, this may be the case when the LO signal is low). That is, a change of data has no effect until the cell becomes transparent for the LO again (for example, high). It is beneficial to change the data when the LO is non-transparent because this automatically makes sure that an undesirable yet sometimes unavoidable delay or mismatch between LO and data is absorbed by the time it takes by the LO to cause transparency of the cell again.
In a dynamically signed RF DAC, the sign change is achieved by inverting the LO signal, and data and LO have to be kept synchronous and aligned also when the LO changes sign to become an inverted LO. The inverted LO has a 180° phase shift with respect to the previous LO. When the sign is changed, data has to be phase shifted by 180° as well in order to preserve the desired mutual timing. Even if this is achieved, however, the cell is transparent for the LO signal during the whole sign-changing transition, and there is no non-transparent timeslot to change data during the sign-changing transition itself. Consequently, during a sign changing event, a mismatch between data and the LO signal can principally not be compensated by performing a change of data during a non transparent period of the LO signal in conventional approaches. Therefore, the spectrum of the generated signal is deteriorated during the sign change. Consequently, there is room to enhance a method for operating radio frequency digital to analog conversion circuitry in the event of a first and a subsequent second input sample with different signs.